Most mammalian tissue contains histamine, concentration being particularly high in the skin, intestinal mucosa and the lungs. Every mammalian tissue that contains histamine, including white blood cells, appears capable of synthesizing the amine from histidine. The principal enzyme involved in catalyzing in vivo the conversion of histidine to histamine is histidine decarboxylase which is specific for the substrate L-histidine. In many tissues the chief storage site of histamine is the mast cell, or in the case of blood, the basophil which is the circulating counterpart of the fixed-tissue mast cell. Mast cells are not the only tissue source of histamine which is present in substantial amounts in the human epidermis, the central nervous system and the gastrointestinal mucosa.
Histamine is involved in various physiological processes. Histamine is released during the antigen-antibody reaction and is responsible, in large part, for the hypersensitivity reaction characterized by vasodilation, itching and edema formation. This type of antigen-antibody reaction wherein the principal cells involved are mast cells and basophils from which histamine is released is commonly referred to as an immediate hypersensitivity reaction. In addition to antigens, or allergens, histamine is released by many chemical substances, macromolecules, venoms, physical insult, such as heat and other injurious stimuli. Gastric acid secretion is known to be stimulated by histamine. Also, histamine is known to be frequently involved in initiation of sensory impulses evoking pain and itching. It has also been found that histamine levels are high in many tissues undergoing rapid growth, for example, embryonic tissue, regenerating liver and malignant growths.
Correlations between levels of histamine and histidine decarboxylase activity in tissues have been made. In the brain which contains histamine and histidine decarboxylase the turnover of histamine is rapid being augmented by stressful stimuli that also increases histidine decarboxylase activity. Inhibitors of L-histidine decarboxylase, such as, .alpha.-hydrazinohistidine are known to lower histamine concentrations. In rat fetal tissue, wherein high levels of histamine are present, it has been shown that inhibition of L-histidine decarboxylase arrests fetal development.
The effects of histamine and its mode of action are well documented. It is believed that the amine exerts its effect through at least two receptors being classified as H.sub.1 and H.sub.2 receptors. Several agents are known to counter the effects of histamine, however, not all such agents prevent the formation of histamine. For example, classical antihistamines useful in treating allergic reactions are believed to exert their utility by interfering with the binding of histamine with H.sub.1 receptors. Similarly agents useful in countering the stimulant effect of histamine on gastric acid secretion are believed to operate by interfering with the binding of histamine with H.sub.2 receptors.
Agents capable of blocking H.sub.1 receptors find use in treating acute exudative types of allergy, such as, seasonal rhinitis, hay fever, pollinosis relieving the sneezing, rhinorrhea, itching eyes, nose and throat. Such agents are also useful in controlling cough and to a degree find use in treating systemic anaphylaxis and bronchial asthma. Antihistamine agents which act through H.sub.1 receptors are also useful in treating allergic dermatoses, such as acute and chronic urticaria, angioedema, itching pruritides, for example, atopic dermatitis and contact dermatitis, in the control of urticarial and edematous lesions of serum sickness, control of blood transfusion reactions and control of drug reactions attributable to allergic phenomena. Agents which block H.sub.2 receptors are useful in treating peptic ulceration, the Zollinger-Ellison syndrome and other gastric hypersecretory states.
Agents which block the formation of histamine by inhibiting the activity of histidine decarboxylase, for example, .alpha.-methylhistidine and .alpha.-hydrazinohistidine, are reported to be useful in the same manner as antihistaminic agents that are blockers of H.sub.1 and H.sub.2 receptors. Additionally histidine decarboxylase inhibitors are implicated as being useful in the control of certain tumors which are high in histamine content.
The compounds of the present invention wherein R is --C.tbd.CH prevent the formation of histamine by inhibiting the action of histidine decarboxylase rendering said compounds useful in treating pathophysiological conditions which result from histamine. The presently claimed compounds can be used in the same manner and for the same purposes as are compounds that antagonize H.sub.1 and H.sub.2 receptors.